Im having a problem with an AR6400. After a nose dive into the dirt (my Champ copy) the left onboard servo has been malfunctioning. What it does is that servo motor keeps pushing the control rod tab toward the end of the slot, without any input from me. It doesn't stay centered at all and wont respond to any commands from the TX. When the battery is unplugged I can turn the large gear and the tab moves easily up and down the slot. When I took off the large gear and plugged in the battery the servo motor goes full tilt without input. Id be curious to know if there is a fix for this, if not Im seriously thinking of cutting the wires that lead to the servo and using plug in servos on my next project.

The micro servos used in the UM series aircraft are subject to an over-range error.

Power off
Hand wind the gears to center the servo.
Power on

Generally fixes it until the next incident that forces the tiny thing to over-range again.

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There is a repair kit to replace the gear, motor and wiper on the multifunction RX/servo/ESC unit also.

Ive done the hand wind thing but when the power is on the servo will automatically return to it's farthest setting, not the default centre setting when the fingers are off the sticks. Its definitely an electronic rather than mechanical problem.

A failed pot (potentiometer) in the TX can do what you describe. Testing with a different RX can quickly check if the TX is the issue if you don't have another TX. A pot failure in the TX would cause it to drive the same channel to the same position with every RX and servo.

Pot failure means the pot itself or the wires between the pot (in the stick gimbal) and the circuit board.
Pot failures were common in one brand of TX I know of due to an error in manufacturing. The factory routed a wire (other than where the designer indicated) where it got cut by the sticks being moved. Repairing the wire and re-routing fixed it every time.

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And you can carefully pull the servo's white plastic off of the RX board to check for debris between the wiper and the contact surface it slides over. Debris there will have the servo sensing its position as being at one endpoint and it will go to the other.
Looking for pictures of how to remove/replace the servo mechanism... or my old RX with broken antenna and power lead... (repairable when I get around to it)

You didn't state originally it wasn't an RX problem ........Yes, I was aware that the AR6400 is a combo unit....and as such, some what fragile when bounced around.......thus, the question related to the RX aspect..............like others have mentioned, the micro servo units have a history of failure over time and wear.......